1. Field of the Invention
The present invention relates to a magnetometric technique utilizing magneto-optical effects by optical pumping.
2. Description of the Related Art
An optical pumping magnetometer utilizing magneto-optical effects by optical pumping uses a glass cell filled with alkali metal gas (e.g., potassium, rubidium or cesium) as a sensor. A static magnetic field is applied to the glass cell, to Zeeman-split energy level of the alkali metal in the glass cell. Then the glass cell is irradiated with the polarized-status-operated light such as linear polarized light, circularly polarized light or elliptically polarized light, or intensity/phase-operated light such as intensity modulated light or phase modulated light. Then magnetism entered the glass cell is detected by utilizing interaction between the light and the magnetism.
A magnetometer described in Japanese Patent Application Laid-Open Publication No. 2007-167616 discharges a laser beam in space thus irradiates the glass cell. Further, the magnetometer has a structure where the glass cell is accommodated in a magnetic shield (a magnetic shield room or magnetic shield case formed of a material having high magnetic permeability) such as permalloy.
Further, Japanese Patent Application Laid-Open Publication No. 2011-89868 discloses a magnetometer using a fiber cell filled with alkali metal atoms as a part of optical fiber. In this magnetometer, a laser as a light source and optical fiber including a glass cell are closely integrated so as to avoid influence of an unwanted external magnetic field and to accurately measure a magnetic field at a measurement point or in a measurement region.
In an optical pumping magnetometer described in Rev. Sci. Instrum. 77, 113106 (2006), to increase the gas density of alkali metal atoms in the glass cell, heated air is blown into a magnetic shield case having the glass cell, to maintain the temperature of the glass cell.
In the magnetometer described in Japanese Patent Application Laid-Open Publication No. 2007-167616, the optical path of the laser beam is not within a short range. Accordingly, in some cases it is difficult to adjust the laser beam optical path, or even when the optical path is controlled, the optical path is easily shifted since the length of the optical path is long, and re-control is required.
In the magnetometer described in Japanese Patent Application Laid-Open Publication No. 2011-89868, the laser as a light source and the optical fiber including the glass cell are integrated. The structure effectively addresses the above-described problems of the optical path control, the shift of the optical path, the influence of the optical path due to air flow and the like. However, when the glass cell is maintained at a temperature higher than a room temperature and the density of alkali metal gas in the glass cell is increased, the structure where the glass cell is included in the optical fiber has temperature unevenness in the optical fiber by maintaining the temperature of the glass cell. In the optical fiber, the polarization of a laser beam passing inside the optical fiber may be collapsed or the laser beam intensity is fluctuated due to such temperature change. When the status of the laser beam is changed, it is impossible to efficiently excite the alkali metal gas in the glass cell. Further, in Japanese Patent Application Laid-Open Publication No. 2011-89868, a use as a wrap status of the optical fiber including the glass cell is described. Even upon use of the optical fiber in folded status, the above-described a laser beam status is changed.
Further, in the magnetometer described in Rev. Sci. Instrum. 77, 113106 (2006), the glass cell is heated with heated air so as to increase the density of alkali metal gas. The magnetic noise caused by the heating using the heated air circulation employed in this example is less in comparison with that in the heating using an electric heater. However, noise occurs when the laser beam is swayed due to the influence of air flow by the heated air and the magnetic measurement signal is fluctuated. Actually, upon use of an optical pumping magnetic sensor in a room with an air conditioning system, when the optical path has unevenness by air flow due to the air conditioning, the sensing may be influenced.